1. Field of the Invention
The present invention relates to a semiconductor device having an ohmic electrode and a method of manufacturing the same.
2. Description of the Related Art
A III-V compound semiconductor device or various kinds of II-VI compound semiconductor devices including a II-VI compound semiconductor light emitting device often employ an arrangement in which a semiconductor layer is epitaxially grown on a GaAs substrate.
This arrangement requires a process of forming an ohmic electrode on the GaAs substrate. The ohmic electrode having ohmic contact to an n-type GaAs substrate includes an electrode made of Ni and AuGe disclosed in a paper by Sharma et al. in "SEMICONDUCTORS AND SEMIMETALS" Vol. 15, p1. This thesis discloses that when the electrode is manufactured by evaporating AuGe on the GaAs substrate and further evaporating Ni on the evaporated AuGe and thereafter the manufactured electrode is subjected to a heat treatment, during the heat treatment, a contact resistance of the electrode is sharply lowered around 350.degree. C. and have a minimum value when the temperature of the heat treatment is within the range of 400.degree. C. to 450.degree. C.
A paper by Kelly et al. in "ELECTRONICS LETTERS" Vol. 14, No. 4 (1978) discloses an ohmic electrode made of Au-SnNi-Au as the ohmic electrode having ohmic contact to the n-type GaAs substrate. Study of FIG. 1 in this paper reveals that a contact resistance of the ohmic electrode can have a minimum value only at a temperature of 300.degree. C. or higher.
A paper by Aydinli et al. in "J. Electrochem. Soc." Vol. 128, No. 12 (1981) discloses an ohmic electrode made of Au/Ni/SnNi. This paper discloses that when samples of the ohmic electrode are respectively subjected to heat treatments at 232.degree. C., 328.degree. C., and 420.degree. C., no diffusion of metal into the GaAs substrate is observed in a sample subjected to the heat treatment at 232.degree. C. but an ohmic contact is achieved in each of the samples subjected to the heat treatment at 328.degree. C. and 420.degree. C. though each of them does not have a mirror surface on its surface.
A paper by Okuyama et al. in "ELECTRONIC LETTERS" Vol. 28, No. 19 (1992) discloses a II-VI compound semiconductor laser employing an n-type GaAs substrate. This paper discloses that the n-type electrode is made of In.
A heat treatment at high temperature is necessary in order that the In electrode has a satisfactory ohmic contact to the n-type GaAs substrate.
Since, for example, a II-VI compound semiconductor light emitting device can emit short wavelength light, e.g., blue light, the II-VI compound semiconductor light emitting device attracts much attention as a light source which allows high recording density in optical recording and reproduction and allows higher resolution in photolithography. The II-VI compound semiconductor light emitting device is formed by epitaxially growing at least an n-type cladding layer, an active layer and a p-type cladding layer which compose a semiconductor light emitting device, e.g., a semiconductor laser on the n-type GaAs substrate by some proper method such as molecular beam epitaxy (MBE) or the like. If the II-VI compound semiconductor light emitting device is subjected to the above-mentioned heat treatment at high temperature for forming an ohmic electrode with a low resistance in a state that such semiconductor layers are eptaxially grown, then it leads to generation and growth of lattice defects such as stacking fault or the like. Further, such generation and growth lower a light emission characteristic and a lifetime of the semiconductor light emitting device. Therefore, it is desirable to avoid carrying out the heat treatment at high temperature.
In order to avoid such heat treatment, there can be considered a method in which an n-type electrode is formed on the GaAs substrate before the semiconductor layers are epitaxially grown. However, when semiconductor layers are epitaxially grown on the gaAs substrate in such electrode forming method, impurities may be introduced into semiconductors or semiconductor layers may be contaminated with impurities, thereby satisfactory epitaxial growth being prevented. Therefore, it is desirable to avoid such electrode forming method as much as possible.
When the semiconductor layers are epitaxially grown on the GaAs substrate and then a thickness of the GaAs substrate is decreased by planing and grinding the GaAs substrate from its rear surface, it is impossible to employ the method of forming the electrode on the rear surface of the GaAs substrate before the epitaxial growth of the semiconductor layers.
When the ohmic electrode is formed on the n-type GaAs substrate by a heat treatment at low temperature, it is possible to employ some methods, e.g., a method of increasing electron density in the n-type GaAs substrate.
However, when the electron density in the n-type GaAs substrate is increased, defect density in the n-type GaAs substrate is increased.